Category: Technical

We place precious data on these small electronic devices. It’s of no surprise that we wonder if it can all just disappear in the flux of electronic bits, bytes, capacitors, and transistors. It’s not easy to grasp how a USB flash drive can store all of our life’s work and keep it safe. Let’s dive in and figure out if these flash based devices can be trusted.

Just Give Me a Straight Answer

USB flash drives and SSDs are fast and increasingly reliable, and are replacing the older tapes and hard-drives. I recall from the numerous studies, articles as well as just the common understanding between the IT professionals, the average lifetime of a hard-drive was between 3-5 years. As the flash memory reduced in price, the surge of the flash drives and SSDs has increased dramatically. However, the knowledge about their endurance or the lifetime of the flash memory based devices has been lacking.

Many factors directly affect the longevity of the flash memory. Some of those factors are the quality and the type of the flash memory and the controller, and the intended use of the USB flash drive or SSD device. If you want the fastest answer without getting technical or getting bored with the details, the life of the modern USB flash drive or SSD would generally match or exceed the average lifetime of a hard-drive under moderate load – 3-5 years or more.

More Details, Please

We started the discussion on this topic in the previous post. It can be accessed here. Without knowing if there is any interest, we focused mainly on a theoretical time model of constant read/write load on the drive. It was assumed that a USB flash drive would be written to 15% of the time under 24/7 365 days a year load.

Since, this topic seems to perk the interest of a few readers, it was important to add more color into the dilemma of predicting the life and death of a flash memory device.

It’s important to understand that the quality of the flash memory as well as the logic of the flash memory controllers can affect the performance as well as the endurance of a flash memory device such as USB flash drive or SSD. The models that are presented assume that the controller can distribute the writes evenly over the flash memory, and that the quality of the flash memory itself will stand up to the promised write/erase cycles.

Below, are two models for computing the endurance of a flash based device. The first is the time model – given a specific write time each day, how much data can be written to a device and thus how long it will last. The second is the data model – given a specific data size, how long will the device last if the data size is written to the drive each day.

Table Adventure or How to Follow the Endurance Numbers

All of the flash memory lifetimes are computed for the TLC type memory. This particular type memory is becoming the standard for the consumer based flash products. It has earned the clever marketing moniker cMLC.

The topmost table, as mentioned before, depicts the time based model. The bottommost table depicts the data model. Both models assume 100,000 IOPS (Input/Output Operations per Second) for modern SSDs; 60K IOPS for older SSDs, and 2.5K IOPS for the USB flash drives. Additionally, both models assume that the SSD type flash memory is rated to 3K write/erase cycles, whereas the USB flash drive memory is rated to 1K. The actual numbers will vary; however these numbers are close enough for the modeling purposes.

On the very bottom of each table is the “Terabytes per year” measure. This number clarifies how each model differs. In the time model, if the flash memory and the device are fast, then they will be able to write much more data in a given period of time. In the data model, time does not matter, because we are given a fixed data to write each day.

The time model assumes that we’ll be writing on average about an hour per day. In the most extreme example of the time model, given a 256GB SSD (100K IOPS rated drive), you’ll have written 469.92 Terabytes worth of data per year or 751.87 Terabytes over 1.6 years before the drive’s ultimate end. In the slowest example of the time model, given a 256GB USB flash drive (2.5K IOPS rated drive), you’ll have written only 11.75 Terabytes worth of data per year or 250.04 Terabytes over 21.28 years before the drive’s end of life.

Now the data model. You’ll notice right away that the Terabytes per year field is exactly the same for each drive. Here it does not matter how long it takes; you can have the fastest drive or the slowest. What matters is that you have a certain amount of data that you write per day. In case of our model, 1GB worth of data per day was chosen. Therefore, the written data per day will be 1GB * 365 day per year = 365GB per year or rounded 0.36 Terabytes per year.

Outstanding Endurance in USB Flash Drives for a Casual User

The time model’s results are not very appealing; however the time model is more suited for a server, professional CAD, or a audio/videophile who are constantly writing large amounts of data each day. For a casual user or a professional who does not reply on writing significant amount of data per day, the data model is much more appropriate.

The data model shows more than 40 years of life for a 16GB USB flash drive. That’s an outstanding endurance number for any electronic device. It’s comforting to know that even though it’s a tiny device, it can be trusted with many years of service for keeping the data safe.

Conclusion; Just Before You Go

Our time and data models show promising results for the SSDs and the USB flash drives. If you’ve got data to move, the flash memory provides the fastest route available today. In the time model, the life of the drive is only shortened due to the incredible amount of data being written to the drive. In the data model, the casual and some professional users can take comfort in the results. The longevity of the 256GB USB flash drive pushing 700 years is as safe as it gets.

Just before you go, only one note. As the drive fills up with data, the models need to be re-referenced. For example, if you manage to fill up half of your 16GB USB flash drive with data on the first day, you’ll only have the remaining 8GB for writing. Then it would be best to reference the endurance for the 8GB drive to get a closer approximation.

[Updated – March 2013]

Every electronic device has a set lifetime, no matter how reliable it may be. Either the mechanical components of the device fail and/or the electronics fail. It may be great news to the manufacturing companies; however not so much to us – the regular consumers.

The news of failing electronics get worse if you consider storage devices such as hard drives, SSDs, and flash memory. Drive failure frequently means loss of important data and utter disappointment. If there is a chance or restoring the data from the backup, it may take hours or even days. It’s difficult to find reconciliation, unless you’re into target practice.

It’s almost impossible to reliably predict a point of time when a drive will fail. Fortunately, there are some tools to estimate an average lifetime of a storage drive. We’ll focus on the flash memory (NAND) storage in this post, and will leave the hard drives to other blogs. Keep in mind that we’ll show a relatively simple way of computing the life of an average flash drive with typical usage.

Flash memory consists of billions of transistors. Each transistor can store from one bit to three bits

The smallest data that the flash memory can read or write is 4KB worth of data. 4KB = 32768 bits

The smallest data the the flash memory can erase is 512KB – that’s huge!

If the data is overwritten in the flash memory, it must first be erased and only then written.

Each time the data is erased, the life of the transistors involved is diminished.

Each time the data is written/read, the controller on the flash drive interacts with the flash memory. That interaction is referred to as IOPS (Input/Output [operations] Per Second)

Now we can start calculating the life of your flash drive. The usual life of each transistor for erase/write cycle is about 1,000 or less. The reason it’s not higher is because the USB flash memory makers often use MLC or TLC type flash memory. From the lesson #1 above; it is the type that stores 2 and 3 bits respectively per transistor (cell).

The average write IOPS measure for a flash drive is about 2500. However, if you’re geeky enough, you can measure yours with Intel’s iometer tool.

So how do I actually get the life of my drive?!

Previously, we used 256K data block in our calculation instead of intended 4K. The result was not a happy number! With this more appropriate computation, the 4GB gets a reputable 32.36 days of heavy duty usage!

The calculation assumes that 15% (that’s the 0.15 in computation) of the time you’ll be writing to the drive. That’s 15% of the 24 hours per day for those 32.36 days. Also, the calculation assumes that you’ll be overwriting (erasing and writing the data) evenly on the flash drive. These are 32.36 pure and hardcore writing days for a causal and even professional user.

Practically speaking, a lot depends on the usage of your drive and the capacity. Additionally, there is a hidden attribute – a factor of how well the controller on the flash drive distributes the writes. In any case, a 4GB drive can last many years. Increasing your capacity, increases the lifetime of your drive so it’s always a good idea to pick the largest size you can.

So what’s that magic number that is used in the formula, you ask? It’s quite simple 🙂 It’s purely used for conversion purposes. Since we use GBs in the numerator and seconds in the IOPS argument, it’s easier to use the 33.25 conversion factor to simplify computation. You get 33.25 with the following operation: 1,000 cycles X (1,048,576KB) / 31,536, 000 sec [There is 1,048,576KB in 1GB; there is 31,536,000 sec in one year]

So how long will your USB flash drive last? That really depends on many factors. However, there are few things you can remember if you want to extend the life of your drive.

Buy the largest capacity you can afford.

Do research. Choose SLC memory over MLC. And MLC over TLC.

Avoid erasing data.

Even the best technology fails; don’t forget to backup!

Armed with the information, you now can protect your investment better. Most of our valuable work is now stored on hard drives and flash memory. The flash memory is being utilized more and more due to its high speed and the lack of mechanical parts. However, there are important facts that you must know to better guard yourself from data loss.

[Update] We continue and expand our discussion on the endurance of flash memory here.

The question about the flash memory (NAND) grades or tiers have come up on occasion. When I first heard about grading being used for the flash memory, I was not sure what to make of it. I was set to learn more about this intriguing nomenclature and get it all figured out.

The Saga

Without too much thinking, Google was the first obvious choice to easily load up on the subject. However, after the clock turned a half of the dial, it was clear to me that it’s not going to be easy. For some reason, I could not get any authoritative explanations; just opinions and hearsay. This will not do; I must press forward!

Truthfully, after realizing that grading may be just hot air, my enthusiasm has waned. A couple of months later, I decided to ask a couple of USB flash drive suppliers about these grades or tiers or something that can relate to the quality aspect of the flash memory. To my surprise, I got similarly vague answers as my extensive Internet search – Bummer!

It’s claimed that grades from A to D exist. Grade A being the best quality and D the worst.

The tiers are the number equivalent to the grades. Grade A would be Tier 1 and so forth.

When the tiers are being refereed to the manufacturers of the flash memory (e.g. Hynix, Samsung); the reference is to the volume, level of the technological know-how, and the level of advancement in the manufacturing process. Tier 1 is highest.

What they say

Grade A – Original (i.e. just as it came form the original manufacturer’s production line) flash with all the manufacturing identification marks, and passed manufacturing inspections

Grade C – Modified or upgraded flash. The controller on the PCB is modified to report a larger capacity than what really exists on the drive. The USB flash drive reports 64GB; however you only got 32GB of the flash storage.

Grade D – Recycled and/or heavily modified flash. If it’s recycled, who knows how much life this memory has left in it. It reports 64GB; but there is a 1MB worth of the actual storage.

Flash Memory Tiers (1,2,3) – Basically Grade A = Tier 1 and so forth

Manufacturer Tiers (1,2,3) – Members of the Tier 1 are Samsung, Hynix. Tier 2 members are Micron, Nanya, Inotera. The Tier 3 members are Elpida, Powerchip, Rexchip.

So what’s the verdict?

It’s rather difficult to tell for certain. It seems that this terminology has been adopted by the flash memory traders and USB flash drive manufacturers. However, an educated guess can be made when the processor technology is compared to the flash memory technology. (What?)

Even though the technology is different, there are some parallels that can be drawn. After the dies are deposited on the wafer, they are tested for quality. Those dies that have a sufficient number of transistors to pass under the intended specifications, are marked and released from manufacturing. Basically, if you are making a 64GB NAND memory die, and the tested amount of transistors return at 549,755,813,888 bits or more, you got a solid die. The testing will also involve performance characteristics.

Some dies do not pass the test to a point where there is less than the intended amount of working transistors. As in the processor chip manufacturing, these are sorted into a different pile. In the processor world, the failed dies become your economical choice processors (e.g. Intel’s Celeron or AMD’s Duron or some other downgraded processor). A similar process must exist for the NAND dies as well.

I want the best grade!

The best grade, one way or another, is Grade A, Tier 1 or Original Flash Memory. If your source is reliable, this is your choice, if you’re looking for the highest performing flash memory with the potential of being the most reliable.

The Grade B flash is close second. It still passed the manufacturing testing, just not to the intended specifications at the time of production. The capacity of the flash memory is full, and its capacity is fully tested.

The Grade C flash is something that you don’t want, unless you’re looking for the biggest bang for the buck. Hmmm… I cannot really think of a promotional or any drive where this can be used. Why would you want to mark the drive higher than it can actually hold? That’s just mean!

Conclusion

The discovery of the grading and tier terminology was a tedious and sometimes frustrating adventure. I’m sure that more specificity and corrections can be made to the discussion; overall, some certain conclusions can be made.

If you’re ready to spend a little more and need extra performance and possibly reliability from your flash memory, choose Grade A (Tier 1). If a reasonable price, good performance and reliability is required, Grade B (Tier 2) is your choice. Grade C (Tier 3) or Grade D should not be your choice in any situation.

In regards to the flash memory manufacturers, Tier 1 Samsung and Hynix tend to stay on top of the NAND manufacturing world. Being the leaders of the industry and with the reputation for being reliable, they are an easy choice.